1. Field of the Invention
The present invention relates to a signal processing circuit board and a liquid crystal display apparatus and particularly to a signal processing circuit board and a liquid crystal display apparatus equipped with a variable resistor which has a control knob.
2. Description of the Related Art
A signal processing circuit board has been proposed which has a variable resistor (referred to as VR hereinafter) mounted thereon for adjusting the supply potential to an optimum level. For example, such signal processing circuit boards are installed in a liquid crystal panel used as the liquid crystal display (LCD) of a notebook computer or the like.
FIG. 1A is a front view of a liquid crystal panel unit employing a conventional signal processing circuit board. FIG. 1B is a back view and FIG. 1C is a side view of the same. FIG. 1D is a cross sectional view taken along the line D—D of FIG. 1A.
As shown in FIGS. 1A to 1D, the liquid crystal panel unit 1 has a liquid crystal panel 2 of a rectangular plate shape mounted on a front side 1a thereof and a (front) shield sheet 3 of a frame form exposed on the outer edge of the liquid crystal panel 2. The liquid crystal unit 1 also has a pair of signal processing circuit boards 4a and 4b (two printed circuit boards) mounted along a horizontal edge and a vertical edge respectively on a back side 1b thereof. The signal processing circuit board 4a is designed for applying a data signal to each pixel of the liquid crystal panel 2 while the signal processing circuit board 4b is designed for applying a horizontal scan signal to each pixel of the liquid crystal panel 2.
As shown in FIG. 1D, the shield plate 3 is made of a frame-like metal plate having an L shape in cross section. The shield plate 3 has an upright wall 3a thereof which extends along the outer edge of the liquid crystal panel 2 to determine an outer edge of the crystal panel unit 1. As best shown in FIGS. 1A and 1D, the shield plate 3 has a VR adjustable aperture 5a provided in a vertical side 3k thereof and communicated with a notch in the upright wall 3a. As shown in FIGS. 1A, 1B, and 1D, the signal processing circuit board 4a has a VR mounted extension 4c extending from a side thereof. A VR 6a of single-side controllable type is mounted on the mounting side of the VR mounting extension 4c (the front side 1a of the liquid crystal panel unit 1) for adjusting the supply potential to an optimum level.
As shown in FIG. 1D, a light guiding plate 7 is disposed on the back side of the liquid crystal panel 2 have a layers structure. The liquid crystal panel 2 is accommodated with the light guiding plate 7 in a frame-shaped back light chassis 8. A light reflecting sheet 7a is bonded to the back side of the light guiding plate 7 covering the entire surface. As the back light chassis 8 is inserted beneath the back side 3c of the shield plate 3, the outer edge 2d of the liquid crystal panel 2 is protected with the shield plate 3. There is provided a space for the VR 6a between the outer side 8a of the back light chassis 8 and the upright wall 3a of the shield plate 3.
As described, the VR 6a of single-side controllable type is mounted with its control knob 6b at the upper on the mounting side of the VR mounting extension 4c which extends from the side of the signal processing circuit board 4a. 
In action, the single-side control VR 6a of single-side (one-side) controllable type can be adjusted to a desired setting of resistance with its control knob 6b turned by an adjusting tool, such as a screwdriver, inserted into the VR adjusting aperture 5a in the shield plate 3. Using the VR 6a of single-side controllable type, the supply potential can be adjusted to an optimum level from the front side 1a of the liquid crystal panel unit 1.
The VR adjustment has to be conducted for each AC driven crystal panel to set the driving voltage to a desired level. If the adjustment is inadequate, the panel may exhibit flickers, burns, or other visual irregularities.
FIG. 2A is a front view of another liquid crystal panel unit employing a conventional signal processing circuit board. FIG. 2B is a back view and FIG. 2C is a side view of the same. FIG. 2D is a cross sectional view taken along the line E—E of FIG. 2B. FIG. 2E is an enlarged view of a VR mounting extension.
As shown in FIG. 2D, the liquid crystal panel unit 9 has a VR 6d of double-side controllable type, not single-side type 6a, mounted thereon with its control knob 6c exposed from the back side of the signal processing circuit board 4a. As shown in FIGS. 2B and 2D, the VR aperture 5a provided in the shield plate 3 is replaced by a VR adjusting aperture 5b provided in the signal processing circuit board 4a on which the VR 6d is mounted so that the control knob 6c is exposed from the back side of the signal processing circuit board 4a (opposite to the components mounting side.). The other arrangement and functions of this model is identical to the liquid crystal panel unit 1 (FIG. 1).
In action, the VR 6d of double-side controllable type can be adjusted to a desired setting of resistance with its control knob 6c turned by an adjusting tool, such as a screwdriver, inserted into the VR adjusting aperture 5b in the signal processing circuit board 4. Using the VR 6d of double-side controllable type, the supply potential can be adjusted to an optimum level from the back side 9b of the liquid crystal panel unit 9.
As the liquid crystal panel units 1 and 9 are reduced in the thickness and the peripheral marginal area, their structure holds a larger area for display hence matching the down sizing and the light weighing of notebook computers with the liquid crystal display. It is now feasible that, for example, the thickness is less than 8 mm while the effective pixel area covers substantially 90% of the entire panel surface. It is essential that since the VR 6a or 6b is controlled by visual manipulation with the signal processing circuit board 4a mounted to the liquid crystal panel 2, its control knob 6a or 6d is exposed from the front side 1a or the back side 1b of the liquid crystal panel unit 1.
However, the VR 6a of single-side controllable type which has a height of about 1 mm and is lower than the VR 6d of double-side controllable type of about 1.5 mm in height is suitable for minimizing the thickness but has to have the VR adjusting aperture 5a provided in the shield plate 3 (See FIG. 1D).
After the signal processing circuit board 4a is assembled with its VR mounting side (the components mounting side) to face the back side of the liquid crystal panel 2, the VR 6a has to be adjusted through viewing the display side of the liquid crystal panel 2. For exposing the control knob 6b of the VR 6a to be adjusted, the VR 6a is shifted to the mounting location outside the liquid crystal panel 2 or beneath the shield plate 3 before the aperture 5a is provided in the shield plate 3.
Also, since the distance between the back side of the liquid crystal panel 2 and the upper center side of the signal processing circuit board 4a is smaller than 1.5 mm (which is equal to the height of the VR 6d), the VR 6d of double-side controllable type is hardly located in substantially a center region of the signal processing circuit board 4a. This allows the VR 6d of double-side controllable type to be located only beneath the shield plate 3 where there is provided a sufficient room (See FIGS. 2B and 2D).
As the shield plate 3 has the VR adjusting aperture 5a provided therein, the physical strength of its metal material may be declined. Also, as the VR is located at the edge of the signal processing circuit board 4a and the room for its installation is preserve in the limited space beneath the back side of the shield plate 3 (FIGS. 1D and 2D), the back light chassis 8 has to be reduced in the thickness thus declining the physical strength. This is a critical condition when the shield plate 3 of the frame-narrowed structure is minimized in the frame width to provide a generous size of the effective pixel area.
For avoiding the drilling of the shield plate 3 or the thinning of the back light chassis 8, it is desirable to locate the VR beneath the back side of the liquid crystal panel 2. There is no room (depth) for installation of the VR between the signal processing circuit board 4a and the light guiding plate 7 bonded on the back side of the liquid crystal panel 2. Preserving any room for installation of the VR between the two members involves reducing the thickness of the liquid crystal panel 2. As the reduction of the thickness may degrade the optical characteristics the liquid crystal panel 2, it is impossible to develop any room. The VR has to stay beneath the shield plate 3.
Since the control knob 6c of the VR 6d of double-side controllable type is exposed from the back side of the signal processing circuit board 4a, it can readily be accessed by the adjusting tool without difficulty. The VR 6a of single-side controllable type is accessed by the adjusting tool inserting deeply through the VR adjusting aperture 5a and may be injured when being groped by the tip of the adjusting tool.
Accordingly, as the conventional liquid crystal panel units 1 and 9 are limited in the installation of the VR, they may be declined in the physical strength and injured at the VR during the assembling process.